On 01/30, Tejun Heo wrote:>> On Mon, Jan 30, 2012 at 02:03:35PM +0100, Oleg Nesterov wrote:> > Perhaps we can use another system_wq, but afaics WQ_UNBOUND makes sense> > in this case. I mean, there is no reason to bind this work to any CPU.> > See also below.>> I've been trying to nudge people away from using special wqs or flags> unless really necessary. Other than non-reentrancy and strict> ordering, all behaviors are mostly for optimization and using them> incorrectly / spuriously usually doesn't cause any visible failure,> making it very easy to get them wrong and if you have enough of wrong> / unnecessary usages in tree, the whole thing gets really confusing> and difficult to update in the future.

You know, I am a bit suprized. To me, it is the !WQ_UNBOUND case is"special". IOW, I think we need some reason to bind the work to thespecific CPU.

> > > Is it expected consume large> > > amount of CPU cycles?> >> > Currently __call_usermodehelper() does kernel_thread(), this is almost> > all. But it can block waiting for kernel_execve().>> Blocking is completely fine on any workqueue.

I understand. But, the blocked worker "consumes" nr_active/worker.

> The only reason to> require the use of unbound_wq is if work items would burn a lot of CPU> cycles. In such cases, we want to let the scheduler have full> jurisdiction instead of wq regulating concurrency.

I am starting to think I do not understand this code at all. OK,perhaps unbound_wq should be used for cpu-intensive works only.

But why do you think that we should use a !WQ_UNBOUND workqueinstead of khelper_wq? And why "a lot of CPU" is the only reasonfor WQ_UNBOUND?

> * If work items are expected to consume large amount of CPU cycles (as> in crypto work items), consider using system_unbound_wq / WQ_UNBOUND.>> * If per-domain concurrency limit is necessary (ie. the number of> concurrent work items doing this particular task should be limited> rather than consuming global system_wq limit), a dedicated workqueue> would be better.

So I don't understand whether you like the idea to kill khelper_wqand use some system_ wq or not (and fix the bug).

I do not really like the current patch. If nothing else, what ifUMH_WAIT_EXEC request actually needs another UMH_WAIT_EXEC/PROCrequest to succeed?

Tetsuo, we spent a lot of time discussing other problems. Whatdo you think about s/khelper/system/ instead of this patch?